The time correlation methods used in the CVI (Colour Velocity Imaging) technique enable definition of measurements and measuring tools which are adapted to the mechanical characterization of arteries. The prior-art technological background in this field is formed mainly by scientific works describing the circulatory system as well as the means previously used to study it, for example:
McDonald's Blood flow in Arteries, by W. W. Nichols and M. F. O'Rourke, edited by Edward Arnold, London.
A device of the kind set forth is known notably from European Patent Application No. 0 458 384 A1 in the name of applicant which corresponds to U.S. Pat. No. 5,107,840 and is incorporated herein by way of reference. Such a device is preferably used for the echographic examination of blood flows in vessels, notably for diagnostic measurement and display of characteristic physiological parameters of these flows.
An embodiment of the device in accordance with the cited application comprises a first unit for measuring, using ultrasonic echography, the speed V(t,z) of the blood flow as a function of time t and of scanning depth z, the measurement of the speed V(t,z) being independent of the ultrasonic wave used, and also comprises a memory for storing samples of the speed V(t,z). This embodiment is characterized in that it also comprises a combination of first circuits for calculating the instantaneous flow rate Q(t) from said samples of the speed V(t,z) as well as a second unit for measuring the radial speeds V.sub.1 (t,z) and V.sub.2 (t,z) of shift of the two vessel walls diametrically bounding said blood flow, memories for storing the speed values V.sub.1 (t,z) and V.sub.2 (t,z), second calculating circuits comprising a circuit for calculating the local energy wherefrom the fixed echos E.sub.2 (t,z) have not been removed, followed by a circuit for calculating respective thicknesses d.sub.1 =z.sub.4 -z.sub.3 and d.sub.2 =z.sub.6 -z.sub.5 of said walls, formed by a threshold detector with the value E'.sub.0 for the determination of the values z.sub.3, z.sub.4, z.sub.5, z.sub.6, two circuits for calculating respective mean speeds V.sub.1 (t.sub.0) et V.sub.2 (t.sub.0) of said walls for each time value t.sub.0, formed by an adder giving .SIGMA..sub.d1 V.sub.1 (t,z) (or .SIGMA..sub.d2 V.sub.2 (t,z) and by a divider by M.sub.1 (or M.sub.2), M.sub.1 and M.sub.2 being the number of measuring samples on the segment [z.sub.3, z.sub.4 ] (or [z.sub.5, z.sub.6 ]), a circuit for calculating the shift of each wall which is formed by an adder supplying: EQU D.sub.1 (t)=.SIGMA..sub.t V.sub.1 (t.sub.0) and D.sub.2 =.SIGMA..sub.t V.sub.2 (t.sub.0),
and a circuit for calculating the symmetrical shift of the walls which is formed by a subtracter and a divider-by-2 and which supplies, in the form of time samples, the instantaneous radius variation .DELTA.r(t)=(D.sub.2 (t)-D.sub.1 (t))/2 of said vessel, and means for displaying the curves Q(t) and .DELTA.r(t) as a function of time t.
Because notably the flow rate Q(t) and the radius variation .DELTA.r(t) of the vessel of an artery are known simultaneously, various calculations can be performed and several highly useful representations can be realised. Actually, the function .DELTA.r(t) can be considered to be an image of the pressure P(t) prevailing in the vessel in a sense that, the dilation of the vessel being a direct function of the pressure, the increasing and decreasing phases of these two functions are the same in the course of the cardiac cycle. Therefore, in a first approximation it is permissible to equate P(t) with .DELTA.r(t), except for a proportionality factor.
A particularly interesting combined mode of representation of the flow rate and the pressure variation consists in calculating and displaying the curve of the cardiac cycle, parameterised as a function of time and retrocoupled to itself, which is formed by the points obtained by plotting on the ordinates the sample values of the function .DELTA.r(t) and on the abscissae the sample values of the instantaneous flow rate function Q(t), the product Q.times..DELTA.r, being a part of the plane of display, then being homogeneous for one power in a first approximation.
On the basis of the measurement of the surface of the cycle and of the surface subtended by the cycle, notably the calculation of the arterial efficiency for the slice of axial thickness dx of the vessel analysed can thus be deduced.
The efficiency calculation, expressed by power ratios and, over the duration of a cycle or a part of a cycle, by energy ratios, directly reflects the energy lost in the part of the vessel analysed during a cycle and hence the roughness and/or the local constriction of the wall of the vessel; this is a precious tool for the detection of stenoses. Moreover, the representation in a loop as described above reveals, for each type of artery, a very characteristic shape which resembles a "signature" and it is imaginable that on the basis of experience such a characteristic shape could be interpreted by the radiologist as revealing a pathology of the heart and/or the artery analysed.
The use of the representation mode given above, and of given other modes, requires highly accurate determination of the functions Q(t) and .DELTA.r(t).
Among the ultrasonic echographs whereby such a high precision can be achieved and which notably can provide, upstream, a measurement of blood flow speeds which is independent of the ultrasonic frequency, there are those which operate according to the time correlation principle already described in European Patent Application No. 0 225 667 in the name of applicant which corresponds to U.S. Pat. No. 4,803,990 and whose unit for measuring the flow speed comprises a correlation circuit which delivers correlation function values, on the basis of two successive echos, and also a multiplexing/interpolation circuit which supplies an estimate of the speed V(t,z) on the basis of said correlation function values.
In the event of a state of the art as described above, it could be desirable to determine parameters, as a function of time, which are very characteristic of hydrodynamic phenomena occurring in an artery. More specifically, it is proposed to determine the elasticity .gamma. and the pressure P at an arbitrary point of an artery, be it in the first place the mean values .gamma..sub.0 and P.sub.0 of these parameters over the duration of a cardiac cycle and, if possible, their variation in time, i.e. the functions .gamma.(t) and P(t). The elasticity of an artery can be defined by the differential relation .gamma.=dr/dP and is also referred to as compliance. A description thereof can be found in the article "Measure de la compliance arterielle" by Georges Nicod, EPFL (Ecole Polytechnique Federale de Lausanne), Presse et information, in relation to the pressure variations in the artery. The cited article also describes a method of determining the compliance in relation to the pressure. The arterial pressure is continuously measured by a photoplethysmograph which measures the pressure differences at the extremity of a finger. Moreover, the diameter of the artery at the point of analysis is determined by an ultrasonic transducer in the pulsed echo (or echo follower) mode. The application of appropriate dephasing, in relation to the cardiac cycle, between the pressure and diameter sensors enables correction of the phase difference of the two curves obtained and formation of the ratio. This method enables a compliance curve to be obtained which can be expressed in percent of increase of diameter per unit of pressure, related to the pressure. It is thus possible to characterize the behaviour of an arbitrary point of an artery and to deduce therefrom the elasticity of the latter at this point. However, this determination remains approximate because the determination of the blood pressure is not performed locally but always at the same area, that is to say at the end of a finger.
Another article: "Non-invasive measurement of interval diameter of peripheral arteries in cardiac cycle" by MOOSER et al., Journal of Hypertension 1988, 6 (suppl. 4), pp. 179-181, published in Switzerland, reveals how to perform an in vivo high precision measurement of the instantaneous diameter of an artery in a non-invasive manner.
The invention aims to determine the elasticity (compliance) of an artery on the basis of echographic measurements which are integrally performed at a predetermined area of the artery analysed.
It is another object of the invention to determine more accurately the compliance of an artery at a given area of the latter.